Radiation sensitive silver halide emulsions containing one or a combination of chloride, bromide and iodide ions have been long recognized to be useful in photography. Each halide ion selection is known to impart particular photographic advantages. Although known and used for many years for selected photographic applications, the more rapid developability and the ecological advantages of high chloride emulsions have provided an impetus for employing these emulsions over a broader range of photographic applications. As employed herein the term "high chloride emulsion" refers to a silver halide emulsion containing at least 50 mole percent chloride and less than 5 mole percent iodide, based on total silver.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of selected tabular grain populations in photographic emulsions.
The various photographic advantages were associated with achieving high aspect ratio tabular grain emulsions. As herein employed and as normally employed in the art, the term "high aspect ratio tabular grain emulsion" has been defined as a photographic emulsion in which tabular grains having a thickness of less than 0.35 .mu.m and an average aspect ratio of greater than 8:1 account for at least 50 percent of the total grain projected area of emulsion. Aspect ratio is the ratio of tabular grain effective circular diameter (ECD), divided by tabular grain thickness (t).
Although the art has succeeded in preparing high chloride tabular grain emulsions, the inclusion of high levels of chloride as opposed to bromide, alone or in combination with iodide, has been difficult. The basic reason is that tabular grains are produced by incorporating parallel twin planes in grains grown under conditions favoring [111] crystal faces. The most prominent feature of tabular grains are their parallel [111] major crystal faces.
To produce successfully a high chloride tabular grain emulsion two obstacles must be overcome. First, conditions must be found that incorporate parallel twin planes into the grains. Second, the strong propensity of silver chloride to produce [100] crystal faces must be overcome by finding conditions that favor the formation of [111] crystal faces. Methods for the preparation of tabular chloride-containing grains with substantially parallel, major [111] crystal faces have been set forth. For example, Wey U.S. Pat. No. 4,399,215, produced the first silver chloride high aspect ratio (ECD/t&gt;8) tabular grain emulsion. Wey teaches an ammoniacal double jet precipitation technique under prescribed pH (8-10) and pAg (6.5-10) in a conventional gelatin precipitation medium to promote very large, thick tabular grain growth. However, the thickness of the emulsions was substantially greater than 0.35 .mu.m, largely compared to contemporaneous silver bromide and bromoiodide tabular grain emulsions. This was due to the extreme ripening conditions provided by the high pH ammoniacal environment. A further disadvantage was that significant reductions in the aspect ratio occurred when bromide and/or iodide ions were included in the tabular grains.
In another process, Wey et al. U.S. Pat. No. 4,414,306, developed a method for preparing tabular grain silver chlorobromide emulsions. The process relies on careful control of the molar ratio between the chloride and bromide ions. Consequently, the final chloride of the grains produced in accordance with this process is limited to 40 mole percent chloride based on total silver. Thus, this process of preparation has not been successfully extended to high chloride emulsions.
Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated Maskasky I) has shown that aminoazaindene growth modifiers, such as adenine, in combination with specific synthetic peptizers having a thioether linkage can be used to make high chloride, high aspect ratio tabular grain emulsions. The synthetic peptizer is used in place of gelatin to produce emulsion grains which are at least 50 mole percent chloride, having a thickness of less than 0.5 .mu.m and which can be formed in acidic media. The principal disadvantage of this approach has been the necessity of employing a synthetic peptizer as opposed to gelatin-peptizers which are almost universally employed in photographic emulsions.
Further investigations into using grain growth modifiers for preparing high chloride tabular grain emulsions has been conducted. For example, Takada et al. U.S. Pat. No. 4,783,398, employs heterocycles containing a divalent sulfur ring atom which form tabular grains having at least 50 mole percent chloride, aspect ratios between 2:1 and 10:1 in the presence of a conventional gelatin peptizer; Nishikawa et al. U.S. Pat. No. 4,952,491, employs spectral sensitizing dyes and divalent sulfur atom containing heterocycles and acyclic compounds; and Ishiguro et al. U.S. Pat. No. 4,983,508, employs organic bis-quaternary amine salts.
Maskasky U.S. Pat. No. 4,713,323 (hereinafter designated Maskasky II), subsequently extended the technology of Maskasky I to the growth of tabular grains in gelatin. Here, high chloride tabular grain emulsions could be prepared by running silver salt into a dispersing medium containing at least a 0.5 molar concentration of chloride ion, an oxidized gelatino-peptizer employing aminoazaindene crystal growth modifiers. An oxidized gelatino-peptizer is a gelatino-peptizer treated with a strong oxidizing agent to modify by oxidation (and eliminate or reduce as such) the methionine content of the peptizer. The gelatin used in this process must be specially oxidized (with hydrogen peroxide, for example) to reduce the methionine content of the peptizer to a level of less than 30 micromoles per gram. Maskasky specifically investigated the use of a gelatino-peptizer containing 56 micromoles methionine per gram gelatin in the process, but failed to obtain tabular grains. King et al., U.S. Pat. No. 4,942,120, is essentially cumulative, differing only in that methionine was modified by alkylation.
As employed herein, the term "low methionine gelatin" refers to a gelatino-peptizer having a methionine content of less than 30 micromoles per gram of gelatin, as disclosed in Maskasky II. The term "high methionine gelatin" refers to a gelatino-peptizer having a methionine content of greater than 30 micromoles per gram of gelatin.
Tufano et al. U.S. Pat. No. 4,804,621 discloses processes for the precipitation of high aspect ratio tabular chloride-rich emulsion grains, using a narrowly defined class of grain growth modifiers in a conventional gelatin media. Tufano et al. observed that over a wide range of chloride ion concentrations ranging from pCl0 to 3 (1 to 1.times.10.sup.-3 M) and a wide range of pH levels, ranging from 2.5 to 9, selected 4,6-diaminopyrimidines were capable of promoting the formation of tabular grains. However, Tufano et al. specifically investigated the use of a 4,6-di(hydroamino)-5-aminopyrimidine (specifically, adenine), but failed to obtain tabular grains using these compounds and explicitly excluded the possibility of having an amino substituent present in the 5-position on the pyrimidine ring.
In the industry, the cost of using low methionine gelatin is about 20 percent more than if high methionine gelatin were employed. Additionally, employing adenine as a growth modifier is generally preferred since it is lower in cost and less toxic of a compound when compared to other growth modifiers. It is apparent that a need exists for providing a process for precipitating high aspect ratio tabular grain emulsions in a high chloride environment which utilizes a high methionine gelatin and a 4,6-di(hydroamino)-5-aminopyrimidine, preferably adenine, as a growth modifier.